Traditionally, non-equalized, multimode fiber, laser-based, fiber optic links are designed with high bandwidth optical transmitters and receivers. This ensures that the dominant cause of inter-symbol interference (ISI) is the optical fiber used for connecting the transmitter and receiver, and not the receivers or transmitters. Usually, the longest link length for proper operation is then determined based on a target maximum ISI penalty. In standards-based fiber optic links, the maximum ISI penalty is usually in the range 3 to 4 optical decibels (dB).
At high data rates such as, for example, 10 Gigabits per second (Gb/s), it is difficult to manufacture, with high yield and low cost, laser-based transceivers using the traditional design methodology. There are many reasons for this, some of which are (1) the difficulty and precision required to ensure high quality signal paths within the transceiver, (2) the difficulty in yielding high data rate optical components of the transceiver, and (3) the high cost associated with testing the components of the transceiver.
Recently, newly specified equalization-based, multimode fiber links have been designed to operate on low bandwidth fiber that creates deep frequency notches or nulls in the Nyquist frequency interval. These links are required to be capable of operating over the maximum distances possible. Therefore, the length of the fiber is a limiting factor on the bandwidth of the link. However, because low bandwidth fiber is used for these links, in order to ensure proper operation, these transceivers incorporate very complex equalizers and high bandwidth optical components, which increase the cost of the transceiver. Although these transceivers are expected to be generally cost effective for their intended applications, they are still relatively expensive compared to lower bandwidth transceivers and are difficult to manufacture, yield and test.
There is a growing need for low cost transceivers and fiber optic links that are capable of operating at high data rates, without the length of the fiber being a limiting factor on the bandwidth of the links. To address this need, new methods of designing laser-based links must be developed.